The present invention relates to a sectored antenna system, and more particularly to a sectored array antenna system used in CDMA mobile communication systems, which provides high success rate of softer handoff and high antenna gain.
The growing interest in supporting voice and image services over wireless medium demands increased capacities of mobile communication systems. In current cellular mobile communication systems, the cell site receives and transmits signals in an omni-direction. Digital cellular mobile communication systems divide wide areas into a number of cells, and communications are accomplished between the cell station and a number of mobile stations. Information signals are transmitted and received in analog or digital form through wireless communication channels.
Generally speaking, the wireless communication channel from the cell station to the mobile station is called forward link, while the wireless communication channel from the mobile station to the cell station is called reverse link. The cell station transmits a number of multiplexed signals to mobile stations and receives signals transmitted from a number of mobile stations simultaneously. To accomplish these complex objectives, multiple access methods or multiplexing methods are required.
As multiple access methods, FDMA (frequency division multiple access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access) are available. Furthermore, it is known that in the light of capacity, CDMA method is better than the other multiple access methods. One good reference is xe2x80x9cOn the capacity of a cellular CDMA systemxe2x80x9d, K. S. Gilhousen, I. M. Jacobs, R. Padovani, A. Viterbi, L. A. Weaver, C. Wheatly, IEEE Transaction of Vehicle Technology, vol.40, No.2.
CDMA systems allocate unique pseudo noise code (PN code) to each user and transmit band spread signals. Receivers use the same code as the PN code in order to despread band spread signals.
CDMA systems determine system capacity on the basis of the received amount of interference. The CDMA system manipulates the sectorization method to increase system capacity. As a result, received amount of interference is decreased. A sectorization method requires installing a sector antenna at a cell station that receives signals in a limited directional range. However, increasing the number of users and their expectation for high quality service demands new communication systems with better capacity than conventional systems. More recently, spatial and temporal signal processing using a smart antenna has been proposed as a good technique in increasing system capacity. An array antenna has been proposed as well.
The array antenna is composed of several antenna sensors and constitutes a narrow beam in the direction of signal- source to suppress signals of other users. A typical antenna array consists of a number of antenna sensors coupled together via some forms of amplitude and phase shifting network to generate a single output. Several shapes of the antenna sensors are common. For example, uniform linear, circular, and planar arrays are available.
FIG. 1 is a diagram illustrating the operation of a sector antenna system in CDMA communication systems with a cell divided into 3 sectors. As shown in the figure, an array antenna 10 is installed at each sector and a beam 11 is constituted for communication with each mobile station.
If the type of signal wave projected to the array antenna 10 is assumed as a plane wave, a steering vector is obtained by equation 1.                               a          ⁡                      (            θ            )                          =                  [                      1            ⁢                          xe2x80x83                        ⁢                          Exp              ⁡                              (                                                      -                    j                                    ⁢                                                            2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                      d                                        λ                                    ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                  θ                                )                                      ⁢                          xe2x80x83                        ⁢            Exp            ⁢                          xe2x80x83                        ⁢                                          (                                                      -                    j                                    ⁢                                                            4                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                      d                                        λ                                    ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                  θ                                )                            ⁢                              xe2x80x83                            .                              xe2x80x83                            .                              xe2x80x83                            .                              xe2x80x83                            ⁢              Exp                        ⁢                          xe2x80x83                        ⁢                          (                                                -                  j                                ⁢                                                      2                    ⁢                                          (                                              M                        -                        1                                            )                                        ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    d                                    λ                                ⁢                sin                ⁢                                  xe2x80x83                                ⁢                θ                            )                                ]                                    [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            
where d is the distance between adjacent antenna sensors (DBAAE); xcex is the wavelength of the transmitted signal; xcex8 is an incidence angle xcex8.
The permissible value that the phase             2      ⁢      π      ⁢              xe2x80x83            ⁢      d        λ    ⁢  sin  ⁢      xe2x80x83    ⁢  θ
may assume lies inside the range xe2x88x92xcfx80 to xcfx80. This means that we must choose the spacing       d    ≤          λ              2        ⁢                  xe2x80x83                ⁢        sin        ⁢                  xe2x80x83                ⁢        θ              ,
so that there is a one-to-one correspondence between the value of the incidence angle xcex8 and the phase             2      ⁢      π      ⁢              xe2x80x83            ⁢      d        λ    ⁢  sin  ⁢      xe2x80x83    ⁢  θ
without ambiguity.
FIG. 2 is a diagram illustrating the operational range of a sector array antenna. As shown in the figure, when incidence angle xcex8 lies inside the range between xe2x88x92xcex8m and xcex8m, the maximum value of d is       λ          2      ⁢              xe2x80x83            ⁢      sin      ⁢              xe2x80x83            ⁢              θ        m              .
Here, the incidence angle xcex8m does not exceed   π  2
in maximum.
In mobile communication systems, when a mobile station escapes from the service area of a cell station and gets into another cell, the handoff function enables users to maintain communications. Two methods such as hard handoff and soft handoff are available.
For an analog cellular system, hard handoff is used. In hard handoff, before establishing a new communication channel, the existent link has to be disconnected. Therefore, signal quality gets degraded since communication is lost for a short amount of time when handoff occurs.
On the contrary, in digital cellular system, when a mobile station escapes from the service area of a cell station and moves into another cell area, an additional link is established with a new cell station without losing the existent link with the former cell station. In other words, the handoff process is composed of the following procedures in CDMA mobile communication systems. First, a communication channel with a cell station is established. Second, another communication channel is established with a new cell station without losing the existent communication channel. Third, the existent channel is disconnected. This handoff process is called soft handoff. Main advantages of soft handoff are that the probability of unwanted call termination is low and users don""t even realize if handoff has happened because at least a communication link is kept during the communication.
In addition, when a mobile station is moving from one sector to another sector, similar process to the soft handoff process occurs. This process is called softer handoff. In softer handoff, the receiver at cell station diversity combines received signals and demodulates them.
FIG. 3 is a diagram illustrating a CDMA communication system in a case that one cell is divided into three sectors and a softer handoff occurs. The ranges of transmitted and received beams of each sector antenna are overlapped to perform softer handoff between adjacent sectors. In the figure, a reference numeral 30 is designated to the area where softer handoff occurs, 31 is to the area where beams are overlapped between sector 1 and sector 2, and 32 is to the inside angle between the central axis of sector 1 and the distal side of the overlap area 31 for illustrating that sector 1 and sector 2 are overlapped with the angular degree of 10.
Under the assumption that system capacity is to be guaranteed, studies regarding how to arrange antenna sensors in an antenna array have been performed in order to expedite softer handoff process.
The present invention provides a sectored array antenna system for CDMA mobile communication systems, which has the optimal distance between antenna sensors.
Advantages of the present invention are that success rate of softer handoff and antenna gain is increased and power consumption of terminals may be evenly distributed. In CDMA mobile communication systems, a cell is divided into a number of sectors. The present invention provides a sector antenna system, which is composed of a number of linear array antennas and distance between adjacent antenna sensors is determined in consideration of the overlapping angle of beams generated by softer handoff. The present invention may also include carrier demodulators, matched filters, a weight vector estimator, weight vector multipliers, weight vector combiners, parallel demodulators, and a signal combiner.
In accordance with the present invention, distances between adjacent antenna sensors are determined by the following equation:       0.4    ⁢    λ    ≤  d  ≤      λ          2      ⁢              sin        ⁢                  (                                    π              S                        +                          θ              S                                )                    
where d is the distance between adjacent antenna sensors, xcex is the wavelength of the transmitted signal, S is the number of sectors, and xcex8s is beam overlapping angle.